1. Field of the Invention
The present invention relates to a light diffusing device. Particularly, the present invention relates to an improved light diffusing device useful for every illuminating a relatively large area at a significantly reduced optical loss.
2. Description of the Related Art
Various known light diffusing devices are useful as a plane light source for transmitting light received from a light source to a desired object to evenly illuminate a relatively large area. These light diffusing devices are practically employed as an illumination means for display devices, for example, advertising display devices, and further, as rear face illumination means for liquid crystal display devices.
Also, various known light diffusing plates, for example, frosted glass plates and milk (opal) white glass plates, are used for light diffusion. The light diffusing plates are usually utilized in such a manner that a light source is located by and spaced from a rear surface of the light diffusing plate at a predetermined distance.
In recent types of liquid crystal display devices, however, it has been attempted to arrange the light source in the vicinity of an edge face of a light diffusing plate which has been utilized as a light incident face, to minimize the size of the display devices. In this attempt, it has been found that, where the light source is merely arranged at the edge face of the usual light diffusing plate, for example, a frosted glass plate or milk white glass plate, it fails to evenly illuminate the entire surface of the light diffusing plate.
To eliminate the above-mentioned disadvantage, U.S. Pat. No. 4,059,916 discloses a concept wherein a rear surface of a light transmitting plate is matted (roughed) in a hairline mode and a light reflecting layer is formed on the mat rear surface so that light irradiated from a light source can be transmitted through the light diffusing plate in an enhanced directivity, and the thickness of the light transmitting plate is gradually decreased from the edge face facing the light source toward the opposite edge face of the plate, so that the brightness at the opposite edge face of the plate is increased and, thus, the brightness throughout the entire surface of the plate becomes uniform.
The above-mentioned type of light diffusing plate is, however, disadvantageous in that the production of the light diffusing plate can be attained only by very fine, delicate and precise work on a surface of a transparent light transmitting plate, and this work causes an extreme increase in the production cost. This is disadvantageous since, from a commercial viewpoint, the cost for producing the light diffusing plate must be reduced. Therefore, it is clear that the above-mentioned type of light diffusing plate is still unsatisfactory in view of the cost thereof.
To remove the above-mentioned disadvantage, Japanese Unexamined Patent Publication (Kokai) No. 61-55684 discloses an improved type of light diffusing device comprising a base plate composed essentially of a transparent light transmitting layer corresponding to a substantially major portion of the plate, a light diffusing layer formed on at least a front surface of the transparent light transmitting layer, and an intermediate layer formed between the transparent light transmitting layer and the light diffusing layer and having intermediate light transmitting and diffusing properties between those of the light transmitting and diffusing layers, at least one edge face of the base plate being a light incident face; and a light reflecting face layer formed on a rear surface of the base plate.
The above-described light diffusing device is significantly advantageous in cost for producing the device.
However, in view of the recent demand for a display on a large screen having a size of 10 to 14 inches, there is an increased demand for the provision of an even further increased uniformity (evenness) in brightness over the entire surface of such a large screen.
Japanese Unexamined Utility Model Publication (Kokai) No. 61-104,490 discloses a new type of light diffusing device wherein a light diffusing plate is arranged on the above-mentioned base plate to improve the uniformity of the brightness thereof.
In the above-mentioned type of light diffusing device wherein light is introduced from a light source into the device through at least one edge face of a transparent light transmitting layer, there is a problem in that a brightness at a location close to the light incident edge face is different from that at another location far from the light incident edge face. Therefore, it is strongly desired to increase the uniformity of the brightness throughout the entire front surface of the device.